1. Field of the Invention
The present invention relates to an optical recording medium, a holographic recording and/or retrieval method, and a holographic recording and/or retrieval apparatus, and more particularly to an optical recording medium having a holographic memory, a holographic recording and/or retrieval method, and a holographic recording and/or retrieval apparatus that can record three-dimensional images, two-dimensional images, or digital data pages.
2. Description of the Related Art
Two-dimensional memories typified by digital versatile disk (DVD) are used as high-capacity and high-density recording media. These two-dimensional memories have been made highly dense by making recording laser wavelength shorter, and reducing a laser spot used to record data by increasing the numerical aperture (NA) of an objective lens used with a pickup. Presently, research and development of two-dimensional memories with violet laser as a light source are being actively conducted.
However, since there are no suitable optical materials in ultraviolet areas and no suitable optical materials used for recording media and lens, it is conceivable that violet laser has the shortest possible recording laser wavelength, which cannot be shorter. A proposed method of increasing NA is that a solid immersion lens (SIL) to reduce a focusing spot by using a hemispheric prism having a high refractive index is used to increase the numerical aperture. This method utilizes evanescent light formed on a prism bottom to form a minute focusing spot. Since the evanescent light is non-propagative light existing locally in the vicinity of a prism bottom and exists only within an area of up to a wavelength from the emission end of SIL, a recording medium must be placed in the immediate vicinity of the prism bottom for recording and retrieval. Therefore, there are many problems to be solved, such as control of the distance between a recording medium and the prism, and the establishment of portability of recording media. The refractive index of prism materials is not more than 2 and recording density is also not increased beyond about four times.
For these reasons, current two-dimensional optical memories reach a limit in terms of recording density. High-density recording of 50 GB or more requires that information be recorded three-dimensionally, including the depth direction of a recording medium (volumetric recording).
Holographic memories recording information in the form of hologram are three-dimensional optical memories that are capable of large-capacity recording. The holographic memories are page-oriented memories that have the high-speed data transfer rate. For this reason, the holographic memories are in the limelight as recording media of the next generation.
Holography is a technique for recording information of the amplitude (intensity) and phase of light in a medium and retrieving it. Coherent light such as laser light is applied to an object, and reflected light (object light) from the object is irradiated onto a recording medium along with another piece of coherent light (reference light), whereby an interference pattern is formed on the recording medium. A light intensity distribution produced by the interference is recorded in a medium as changes of refractive indexes and/or absorption coefficients, and a thing produced in the medium is referred to as a hologram. If reference light only is incident on the recording medium in which the hologram is recorded, the hologram functions as diffracted gratings, so that object light is retrieved.
In the holographic memories, digital data (binary data of 0 or 1) is converted to on/off (bright/dark) patterns using an spatial light modulator and incident on a recording medium as object light, whereby the digital data can be recorded as a hologram. Reference light is applied to a recording medium to reproduce object light and the reproduced object light is received by a photodetector for photoelectric conversion, whereby original binary data can be retrieved from the obtained electric signals. Recently, research into the digital holographic memories is in progress from the engineering point of view, as found in S/N and bit error rate evaluation based on a specific optical system of the digital holographic memories and a volumetric multiple recording method, proposal of two-dimensional encoding, and a study of influence of aberration of an optical system.
As holographic recording materials, attention is being given to polymer materials that are inexpensive and can be easily formed to disk shape. Vigorous research is being done on so-called photopolymers for use as ROM media, and for use as erasable media, photosensitive polymers containing photoisomerizable groups such as azo groups are promising.
To achieve a large capacity by holographic memories requires that the thickness of a recording layer in which holograms are recorded be increased and plural hologram be recorded within an identical volume. For example, to store 100 GB or more of digital data on one disk, the thickness of a recording layer must be 1 mm or more. However, making the recording layer thick while maintaining optical quality is very difficult and expensive in the present situation.
A method of achieving a large capacity by solving this problem is described in Japanese Published Unexamined Patent Application No. Hei 9-101735. In Japanese Published Unexamined Patent Application No. Hei 9-101735, a recording and/or retrieval method is described which uses an optical recording medium of multilayered optical waveguide type. This optical recording medium has plural optical waveguide layers and recording layers stacked on a substrate through clad layers so that optical waveguide layers sandwiched by adjacent clad layers constitute optical waveguides. This optical recording medium is used to record a hologram by selectively introducing a reference light into an optical waveguide from an end face of an optical waveguide layer, irradiating object light (signal light) from an interface of the optical waveguide layer, and causing evanescent light leaking to a recording layer and the object light to interfere with each other. In this case, the thickness of the recording layer required to record one hologram can be as thin as several micrometers and the film of the recording layer can be produced without impairing optical quality by the spin coating and casting method. Stacking two or more of such thin recording layers enables multiple holographic recording.
However, a recording and/or retrieval method using an optical recording medium of multilayered optical waveguide type as described in Japanese Published Unexamined Patent Application No. Hei 9-101735 has the following problems. In the optical recording medium, a hologram is recorded by guiding reference light used in the recording through a waveguide layer, and causing evanescent light leaking to a recording layer and object light to interfere with each other. However, since the evanescent light reaches a recording layer only as far as wavelength order, a hologram cannot be recorded with sufficient depth in the direction of medium thickness. Also, the evanescent light is too weak to provide sufficient exposure intensity. As a result, the recorded hologram provides no satisfactory diffraction efficiency. Also, during retrieval, it is difficult to apply a reading light of sufficient intensity.
To solve these problems, it is desirable that a hologram is recorded by guiding reference light and reading light through a recording layer and causing reference light guided through a recording layer instead of evanescent light and object light to interfere with each other, and the hologram is retrieved by reading light guided through the recording layer. However, the recording layer is made of a photosensitive material in which the reference light and reading light are absorbed to record holograms. Therefore, if the reference light were introducing into the recording layer from an end face thereof and guided through it, a hologram could not be recorded because of large optical loss. Also, guiding reading light during retrieval would destroy a recorded hologram.